also: よろずとせ
noun
ten thousand years; an extremely long time; eternity
Literally 'ten thousand years'; often used proverbially or rhetorically for great longevity or an unimaginably long time. The reading よろずとせ is archaic.
鶴は千年、亀は万年と言われる。
It is said that cranes live a thousand years and turtles ten thousand.
この歌は国の万年の栄えを祈るものだ。
This song prays for the country to prosper forever.
noun, used as a prefix
perennial; perpetual; ever-present
Only when written 万年 · Only with reading まんねん
Prefix use read まんねん, forming compounds that mean something is permanent, perennial, or always in the same state. In human-status compounds, it can sound mildly humorous or critical, as in 'perpetual substitute' or 'always last'.
山頂には万年雪が残っている。
Perennial snow remains on the mountain summit.
彼はチームの万年補欠だった。
He was the team's perpetual substitute.
Old or obsolete reading for the noun sense; useful mainly for recognizing archaic or literary usage.
Attested old spelling/reading for the noun sense. Do not confuse this with the common word 万歳 read ばんざい.
Means 'eternity' or 'forever' more directly and abstractly; 万年 is literally 'ten thousand years' and is often proverbial or compound-forming.
Means 'many years' and is more literal or formal; 万年 emphasizes an extremely long, lasting, or perpetual quality.
A technical botanical term for a perennial plant; 万年 as a prefix is broader and appears in general compounds such as 万年雪 and 万年補欠.
万 means 'ten thousand' or 'myriad' and 年 means 'year', so 万年 literally means 'ten thousand years'. The old reading よろずとせ reflects native-style words for 'myriad' and 'years'; the exact historical usage of the alternate spelling 万歳 is not detailed.